1. Field of the Invention
The present invention relates to an optical system and an optical apparatus including the optical system. In particular, the present invention relates to an optical system useful as a photographic optical system for an image pickup apparatus, such as a digital still camera, a digital video camera, or a silver-halide film camera.
2. Description of the Related Art
As a photographic optical system used in an optical apparatus, such as a digital camera or a video camera, a conventional middle focal length photographic optical system having a high aperture ratio of about 1.2 to 2.0 in F-number and a relatively long focal length has been used for shooting portrait images and indoor sport scenes. Current market preferences require that the photographic optical system of this type has a short total system length (i.e., the lens total length, which is the length from a surface of a first lens closest to the object plane to the image plane), that the photographic optical system is small-sized, and that the photographic optical system has a high optical performance.
In general, however, if the lens total length of a photographic optical system is reduced, chromatic aberration, such as axial chromatic aberration or chromatic aberration of magnification may increase. As a result, the optical performance may degrade. In particular, in a middle focal length telephoto lens or a long focal length telephoto lens, chromatic aberration may increase as the focal length is increased.
On the other hand, in a photographic optical system having a high aperture ratio and high brightness, the depth of field is shallow and the various aberrations greatly affect the image quality. Accordingly, if a photographic optical system like this is used, it is necessary to correct aberrations with a high accuracy.
As a conventional optical system, a middle telephoto photographic optical system having a high aperture ratio and a relatively long focal length has been used. Japanese Patent Application Laid-Open No. 05-188286 discusses a photographic optical system that uses a lens made of a middle dispersion glass material having a high refractive index to correct various aberrations, such as spherical aberration and curvature of field.
Furthermore, a telephoto lens, which has a front lens group including a lens made of an anomalous partial dispersion material, such as fluorite, to correct chromatic aberration, has been used. A telephoto lens discussed in U.S. Pat. No. 7,411,745 uses a lens made of a low dispersion material having a low refractive index to correct chromatic aberration.
A middle dispersion material having a high refractive index, which has an Abbe number of about 40 to 60 and a refractive index of about 1.6 to 1.8, generally has negative anomalous partial dispersion. Accordingly, if a lens made of the middle dispersion material is used as a lens having a positive refractive power, axial chromatic aberration may increase.
A low dispersion material having a high refractive index, which has an Abbe number of about 70 to 90 and a refractive index of about 1.4 to 1.5, generally has positive anomalous partial dispersion. Accordingly, if a lens made of the middle dispersion material is used as a lens having a positive refractive power, axial chromatic aberration can be appropriately corrected. However, because the refractive index is low, it is required to increase the curvature of the lens surface (a refractive surface) to achieve a desired level of refractive power. Accordingly, spherical aberration and curvature of field may easily occur.
Therefore, it is necessary to appropriately correct chromatic aberration and various aberrations, such as spherical aberration and curvature of field at the same time by appropriately setting the parameter in the material of the lens to achieve a high optical performance for the entire image plane. In particular, if a middle focal length lens, which has a high aperture ratio and a relatively long focal length, is used, it may become difficult to achieve a high optical performance for the entire image plane unless chromatic aberration and various aberrations, such as spherical aberration and curvature of field, are appropriately corrected.